Oh and another idea to go with the above is CAC and Smart Cards. http://en.wikipedia.org/wiki/Common_Access_Card
(Another Wikipedia link but I didn't think it would be smart to link everyone to US milatery sites) for individual accounts or smaller than huge corporate accounts. http://www.smartcardbasics.com/smart-card-types.html
A CAC is basically a Smart Card this being scifi you can play with the concept a little.